Ignore build-related files in "git archive" (fixes #8167)

See http://open.silverstripe.org/ticket/8167

This is useful for keeping standard "composer create-project"
checkouts clean. Unless they use "--keep-vcs", in which case
.gitattributes is (correctly) ignored. 

This has the side effect that archive-checkouts can't
run phing or phpunit scripts, but I think that's
acceptable.

Will also affect the github.com downloads by the way.